The present invention concerns new electrolytes for use in galvanic cells, in particular lithium cells. In this paper a lithium cell is defined as follows: Essentially a lithium cell consists of an anode, a cathode, and an electrolyte. The anode consists of lithium, a lithium alloy, or a compound which is able to intercalate lithium ions, for example carbon. In general the cathode consists of a substance which is able to intercalate lithium ions or reacts electrochemically with lithium ions, where the potential of the cathode is markedly higher than the potential of the anode. The electrolyte consists of one or more salts which are dissolved in a suitable solvent, a mixture of solvents, a polymer, or a mixture of a polymer with one or more solvents. A distinction is made between primary and secondary lithium cells.
This definition includes the so-called `rocking chair batteries`. Recent investigations in this field are described in a review of B. Scrosati (J. Electrochem. Soc. Vol. 139, 2776, (1992)).
Typical solvents currently in use include:
Organic carbonates such as propylene carbonate (PC) or ethylene carbonate (EC). Linear and cyclic ethers and polyethers such as dimethoxyethane (DME), diethoxyethane (DEE), tetrahydrofurane (THF), 2-methyltetrahydrofurane (2-Me-THF), dioxolane, and the polymer polyethylene oxide as well.
Typical salts currently in use include:
Lithium perchlorate (LiClO.sub.4), lithium tetrafluoroborate (LiBF.sub.4), lithium hexafluorophosphate (LiPF.sub.6) lithium hexafluoroarsenate (LiAsF.sub.6), lithium trifluoromethylsulfonate (LiCF.sub.3 SO.sub.3), and lithium bis(trifluoromethylsulfonyl)imid (Li(CF.sub.3 SO.sub.2).sub.2 N).
The above-mentioned salts show some serious drawbacks. Lithium perchlorate has a tendency to explode in combination with some solvents such as dioxolane. Compounds with fluorinated inorganic anions, such as lithium hexafluorophosphate, are of low thermal stability, and generate Lewis acids by dissociation which are able to polymerized the solvent. Because of its arson content lithium hexafluoroarsenate is environmentally damaging, and generates carcinogenic by-products by reaction with lithium. Lithium bis(trifluoromethylsulfonyl)imide is comparatively expensive.